Method for drying paper

ABSTRACT

In a process for drying paper, in particular whole books but also other absorbent and in particular organic materials present in particular in layered or laminated form, to achieve residual moisture contents of 1% and below in the shortest possible time and without other disadvantageous effects on the paper or the books, in particular as a first treatment step in the mass deacidification of paper, the paper is heated by being acted upon with warm air and dried by being acted upon with reduced pressure. The operation of acting upon the paper with reduced pressure is effected periodically.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention concerns a process for drying paper, in particular wholebooks. This drying process however can also be applied to otherabsorbent and in particular organic materials which occur in particularin layered or laminated form.

From time to time, paper and in particular paper in the form of bookshas to be dried relatively quickly and down to very low moisturecontents of in part below 1% by weight—% of water.

That is necessary when books which have been stored in an excessivelywet condition or which have become wet due to water damage are to berescued, in which case the drying operation into a range of somewhatbelow 10% moisture content is implemented by means of conventionalmethods, that is to say by producing a flow of warm air therearound,storing the books in well-heated and well-ventilated drying chambers,and so forth.

Further drying to about 1% residual moisture content and below isnecessary however in particular in regard to what is known as massdeacidification of paper as a first treatment step or as a terminatingdrying operation (post-drying), in which the books as a whole, that isto say without previously being separated into individual sheets orleaves, which is an expensive and complicated operation, are chemicallydeacidified in order to eliminate the acids which ultimately destroy thebooks, such acids being mostly contained in the paper size but also inthe inks and printing dyes.

In those comparatively advantageous processes for preserving inparticular historical stocks of books, at a low level of financialexpenditure, the operation of drying the books to a residual moisturecontent of below 1%, which is necessary before implementing the chemicaltreatment, represents the bottleneck in the process, in terms of timeand capacity, as such a drying operation can only be implemented bymeans of heating and subjecting the books to a reduced pressure, and canthus be carried out in so-called autoclaves, that is to say containerswhich can be hermetically closed shut and which can be acted upon by areduced pressure.

In order in that procedure to achieve the maximum quantitativethrough-put of paper, that is to say for example books, the books ofwhich a conventional size of autoclave will accommodate for example 100kg are initially dried by microwave drying in the autoclave and themoisture which is so liberated is removed by the subsequent applicationof vacuum. This cycle which is only implemented once takes up arelatively small amount of time as the drying operation using microwavesonly requires between about 2 and 3 hours.

The disadvantage of this process however is that even very small metalconstituents in the book resulted in the book burning in the area aroundthose metal constituents. Since even the relatively small amounts ofheavy metal in historic inks or in book-printing inks and dyes werefound to be sufficient as metal constituents in that sense, with thepassage of time, microwave drying is now practically no longer used.

Other drying methods such as for example drying using infra-redradiation also raised other problems, for example a variation in colorat least in the regions of the books which are near the surface, butalso an insufficiency of action in terms of depth, with the consequencethat the regions in the middle of a book could not be adequately dried.

Therefore, drying by means of warm or hot air is preferred, as thisappears to be the only procedure which does not seem to involvediscernible disadvantages, irrespective of the age of the book andtherewith the nature of the paper, the composition of the printing dyeor inks and so forth.

In that respect the treatment using hot or warm air has hitherto alwaysbeen implemented in such a way that the books are disposed in anautoclave whose walls are heatable. By the walls of the autoclave beingheated, the air in the interior of the autoclave is heated and the paperdisposed therein is thus also heated, by way of the air. In part theincrease in temperature of the paper was also effected directly by wayof radiant heat which was caused to act directly on the books, from theheating elements of the wall of the autoclave.

During the heating operation the interior of the autoclave and thereforealso the books therein is permanently subjected to the action of reducedpressure, and otherwise the autoclave is closed. Therefore in particularno fresh air was introduced into the autoclave during the heating andsuction removal procedure.

That mode of operation prevented freshly entering air from having ahigher moisture content than the residual moisture content that it wasendeavoured to achieve in the paper.

The disadvantage here however is that between 1 and 2 days are requiredto dry a batch of between 80 and 100 kg of books of initially between 6and 8% by weight of water to below 1% by weight of water.

Therefore the object of the present invention is to provide a processfor drying paper, even in compact form such as for example whole books,in which residual moisture contents of 1% and below can be achieved inthe minimum possible time and without other disadvantageous effects onthe paper or the books.

That object is attained in that the treatment with reduced pressure iseffected not permanently but periodically. In addition, new, heated, dryair is fed to the paper at least between the phases of thereduced-pressure treatment, with the moisture content of that new airbeing below 1% by weight of water.

In that situation heating is effected by using warm air and drying iseffected by means of the application of vacuum in a plurality of cyclesin succession, preferably in between four and six cycles.

The efficiency of this process is very good by virtue of the fact thatthe moment in time from which further heating or further drying becomesinefficient can be very clearly established.

First of all, for the heating phases, it is necessary to establish amaximum temperature which should be as high as possible in order tocause as much moisture as possible to diffuse out of the paper.Generally that procedure is implemented using a temperature of between40 and 50° C. as no residual adverse effects on the paper or theprinting on the paper are still to be feared at that temperature, suchas for example discoloration phenomena, excessive brittleness and inthat case breakage in the subsequent treatment of the paper and soforth, irrespective of the epoch from which the paper to be treatedoriginates.

The duration of the heating phases is determined by the considerationthat the desired final temperature of for example 45° must be achievedin the interior of the amount of paper disposed in the autoclave, thatis to say for example in the interior of the books which are stored inthe center thereof. In that respect the dried warm air which acts on thepaper should be not substantially hotter but only about a maximum of 10°C., in particular only a maximum of 5° C., hotter, in order not to causean excessive rise in temperature of the outside regions, before thefinal temperature in the interior of the batch of paper is reached.

After the final temperature has been reached even in the center of thebatch of paper, the feed of warm air is interrupted and the autoclave isclosed except for the reduced-pressure connection, by way of which thepaper is subjected to the action of compressed air.

Depending on the power of the vacuum pump connected thereto, only a fewminutes are required when dealing for example with 100 kg of paper, inorder to achieve the desired reduced pressure of less than 10 mbar andpreferably less than 1 mbar.

However the holding time of that reduced pressure is a crucial aspect interms of drying the paper:

When the reduced pressure is applied to the autoclave, the temperatureof for example 45° C. which is initially present in the interior of theautoclave begins to fall rapidly towards ambient temperature outside theautoclave, as in fact a further supply of heat in the form of preheatedair no longer takes place.

Very suddenly however, that is to say after a holding time of at leasthalf an hour, within between two and three minutes and preferably evenwithin less than a minute, that temperature drop slows down drasticallyor comes to a stop, or is even reversed to become a rise in temperature.

The cause of the stagnation in the fall in temperature is that, below agiven temperature (in dependence on the degree of drying which hasalready been achieved), no further evaporation worth mentioning occursand thus the evaporation enthalpy becomes equal to zero.

After the suction flow is broken off, the temperature within theaccumulation of paper which had been previously heated to about 45° C.is equalized and is set to a temperature level which self-evidently ishigher than the temperature in the approximate vacuum surrounding theaccumulation of paper and prevailing within the autoclave. As thattemperature of the vacuum was previously measured at the temperaturesensor disposed in the accumulation of paper, an increase in temperatureis displayed there.

As soon as this effect of slowing down the fall in temperature ortemperature stagnation or even the rise in temperature occurs, theinvention provides that application of the vacuum is broken off and thepaper is again heated by acting thereon with pre-dried warm air. In thatphase of operation the autoclave can be closed and warm air iscontinuously urged into the closed space, which however results in onlya low level of air exchange. Preferably the autoclave is at leastslightly opened at another location in order to cause the pre-dried warmair to flow through the interior of the autoclave, in which respect flowrates of between 0.1 m/s and 2.0 m/s, in particular between 0.5 m/s and1.0 m/s, should prevail.

That procedure avoids causing vacuum to act for an unnecessarily longperiod of time and/or causing warm air to act for an unnecessarily longperiod of time, while permitting a high quantitative through-put in thedrying operation, combined with a level of energy consumption which isat an optimum low figure.

Accordingly, with between 6 and 8% initial moisture, it is possible toachieve a residual moisture content of below 1% in a maximum of one anda half days, generally in between 12 and 24 hours, for which purposeapproximately five and in some cases even only four cycles consisting ofheating and drying have to be effected.

In that respect, with an accumulation of paper of for example 100 kg,the first heating phase lasts between about 2 and 3 hours and theduration of those heating phases is slightly reduced to between 1 and 2hours in regard to the last heating phase.

In contrast the evacuation phases become longer from one cycle toanother and in particular they double from one cycle to another.

With the specified amount of paper of 100 kg, the first evacuation phaselasts for between 0.7 and 0.8 hours and the evacuation phase after thefourth heating phase lasts about 6 hours. After the fifth heating phaseevacuation is then generally effected over a period of between five andten hours, whereby approximately between the last 0.5 and 0.7% by weightof water, with respect to the mass of paper, is achieved, beforereaching the final content of less than 1.0% by weight of water.

A renewed heating operation within that long last evacuation phase is inthat respect no longer meaningful, in consideration of the followingtendency:

The reversal in trend which is to be observed during the vacuumtreatment, that is to say a reduction in the rate of the fall intemperature or even stagnation and a reversal into the oppositedirection, admittedly takes place in terms of time after an increasinglylong period of vacuum treatment, but in regard to temperature, attemperatures which are ever increasing, although during the heatingphases the same final temperature is always attained in the interior ofthe accumulation of paper, for example 45° C.

If, after the for example fourth heating phase, upon subsequentevacuation, the reduction in the rate of drop in temperature occursafter an evacuation time of about one hour, in that case a temperatureof about 40° C. still generally obtains in the interior of theaccumulation of paper.

However, as soon as the temperature at which the reversal in trend ofthe drop in temperature occurs is only 5° C. or less below the finaltemperature of the preceding heating phase, renewed heating is then nolonger meaningful, and then the paper is only still subjected to theaction of vacuum until the final state of less than 1% by weight isreached.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment according to the invention will now be described ingreater detail by way of example with reference to FIG. 1. FIG. 1 showsthe temperature pattern in the interior of a relatively compactaccumulation of paper, that is to say for example between the pages of abook which is arranged in the interior of a stack or an accumulation ofbooks.

DETAILED DESCRIPTION OF THE INVENTION

That temperature is plotted against time, thus showing the time durationand the qualitative configuration of the heating phases and theevacuation phases.

This experiment which was conducted with a collection of paper of aweight of between 80 and 100 kg shows that, starting from 20° C. ambienttemperature and with a slight increased pressure, about four hours wererequired in order for the desired final temperature of 45° C. to beattained at the sensor at the interior of the accumulation of paper byblowing air into same or blowing air through same, with the temperaturethereof being about . . . ° C. and containing less than 1.0% by weightof water. The subsequently applied vacuum causes the temperature to fallwithin 0.6 hour to 25° C., in which case temperature stagnation occursalmost abruptly.

As soon as this is the case, the reduced-pressure source is cut off orseparated from the autoclave and the paper in the interior of theautoclave is heated again by means of the above-described warm air untilonce again a final temperature of 45° C. is attained at the temperaturesensor in the interior of the accumulation of paper, with a period ofjust four hours being again required for that purpose.

After that is achieved and the reduced-pressure source is againconnected to the autoclave, the desired reduced pressure of up to 1 mbaris initially built up within the autoclave very much more rapidly, as isshown by the very much more pointed configuration of the peak in thissecond cycle.

The fall in temperature is initially just as rapid, but below about 35°C. it begins to become slower and already reverses at a temperaturevalue of about 32° C. to become a rise in temperature, although 1.4hours are required for that, from the application of vacuum.

The subsequent heating operation from about 32° C. to once again 45° C.takes place markedly more slowly. For this temperature difference whichis still only about 10° C. 3.8 hours are required, when being subjectedto the action of warm air in the same manner.

In the subsequent treatment with vacuum, almost the same configurationas after the second heating phase is to be observed, and also thesubsequent heating operation with which approximately the sametemperature difference as a cycle previously must be implemented alsorequires approximately the same time.

After this fourth heating phase the initially sharp drop in temperatureagain reverses after about one hour to become a rise in temperature, butthis already occurs at a temperature which is still 40° C.

Then, reduced pressure is applied in an evacuation phase of prolongedduration on a continuous basis, but at least more than two hours, insome cases up to 16 hours, until the desired residual moisture level isreached.

What is claimed is:
 1. A process for drying an organic material,comprising: heating the material with warm air having a moisture contentbelow 1% by weight of water; and periodically drying the material with areduced pressure.
 2. A process as set forth in claim 1, whereintreatment of the material with warm air occurs periodically andalternately with the treatment by means of reduced pressure.
 3. Aprocess as set forth in claim 1, wherein the treatment with warm air iseffected by transporting air that has already been previously heated tothe material and passing it over the material.
 4. A process as set forthin claim 1, wherein when the material is acted upon with warm air a flowrate of the air in the proximity of the material is between 0.1 m/s and2.0 m/s.
 5. A process as set forth in claim 4, wherein the flow rate isbetween 0.5 m/s and 1.0 m/s.
 6. A process as set forth in claim 1,wherein the treatment with warm air and the subsequent treatment withreduced pressure is effected in between four and six successive cyclesin order to dry between six and eight % by weight of moisture to amaximum of 1% by weight of moisture.
 7. A process as set forth in claim1, wherein in the treatment by reduced pressure, the reduced pressure isless than 10 mbar.
 8. A process as set forth in claim 7, wherein thereduced pressure is less than 1 mbar.
 9. A process as set forth in claim1, wherein treatment of the material is effected batch-wise.
 10. Aprocess as set forth in claim 9, wherein said treatment is effected inan autoclave.
 11. A process as set forth in claim 1, wherein thematerial is acted upon with warm air to a final temperature for thematerial of between 40° C. and 50° C.
 12. A process as set forth inclaim 11, wherein the final temperature is about 45° C.
 13. A process asset forth in claim 1, wherein when treating an amount of 100 kg ofmaterial the first heating phase lasts between about 2 and 3 hours andthe duration of the heating phases gradually decreases to between about1 and 2 hours in the last phase.
 14. A process as set forth in claim 1,wherein the action of reduced pressure on the material after therespective heating phases is effected until the temperature of thematerial which falls sharply during the treatment with reduced pressurevery greatly slows down.
 15. A process as set forth in claim 14, whereinthe fall in temperature even reverses to become a rise in temperature.16. A process as set forth in claim 1, wherein in the treatment of 100kg of material the first evacuation phase lasts between 0.7 and 0.8hour.
 17. A process as set forth in claim 1, wherein the duration of theindividual evacuation phases approximately doubles from one cycle toanother.
 18. A process as set forth in claim 17, wherein the duration ofthe evacuation phase is about 6 hours after a fourth and last heatingperiod.
 19. A process as set forth in claim 1, wherein the organicmaterial is paper.
 20. A process as set forth in claim 1, wherein theorganic material is a book.
 21. A process as set forth in claim 11,wherein the organic material is a book and the final temperature is inan interior of the book.